Continuously operating centrifuge



United States Patent [72] Inventor Dirk l-loks Hengelo, Netherlands [21] Appl. No. 779,960 [22] Filed Nov. 29, 1968 [45] Patented Nov. 3, 1970 [73] Assignee Stork-Werkspoor-Sugar N.V.

l-lengelo, Netherlands [32] Priority Dec. 2, 1967 [33] Netherlands [31] 6716436 [54] CONTINUOUSLY OPERATING CENTRIFUGE [50] Field of Search 2 [0/78,

[56] References Cited UNITED STATES PATENTS 1,235,193 7/1917 Elmore 210/374 2,779,473 1/1957 Van Wezel 210/374 Primary ExaminerJ. L. DeCesare A tt0rney- Snyder and Butrum, lmirie and Smiley 11 Claims, 6-Drawlng Figs.

[52] U.S.Cl. 210/374,

, 210/380 [51] lnt.Cl. .B04b1l/08 A6 1 II 18 4 l2 5 t 15 /6 7 2a 17 Patented Nov. 3, 1970 3,537,591

Shoot of 2 FIG] 38 1,2 37 I 1 l2 5 1 I I8 32 ,7 7 27 I; as I? A [I N I I h g I 3 ii :4 m

INVENTOR DIRK Hons ATTORNEYS Patented Nov; 3, 1970 Sheet i of 2 IIII-I-IIII n ...l. 1-1

INVENTOR DIRK HOKS ATTORNEYS '1 CONTINUOUSLY OPERATINGCENTRIFUGE The invention relates to a continuously operating centrifuge for separating a given mixture of solid constituents and liquid,

the drum so that a considerable amount of power is required to transport themateriala In a known device, the attempt has been made to transport, the materialtby means of powerful jets of a gaseous medium. The power required in the compressor to deliver the gaseousmedium at pressureis very high, so that the centrifuge becomes expensive.

The object of the invention is to'providea centrifuge of this type in which the above-problem is solvcd-ina simple manner. To this end, aeeordingto the invention, thewall of the drum in a plane through the axisof rotation forms such an angle with said axis thatthe tangent of the said angle .is substantially equal to the coeffieient of friction'betweenthe drum surface and the cake which is required to be formed thereon and which consistsmainlyof the solid tconstituents so that in operation that component of thecentrifugal force acting on the cake ,whichis direeted along the drumsurface is substantially cancelled out by.the;frictional.forcedirected in opposition thereto,andthescraping device is provided with means which, during operation, disturb theequilibrium between the forces acting on'the cake in favour of thecentrifugal force component as soon asthe cake is taken from the drum surface by the scraper. lnqa'centrifuge of .thisLtype, the centrifugal force acting on the cakeis used to transport the material lifted off thedrum wallsThusthe force which gives rise to the problem in the known centrifuge is'put'to good use inthis case. The sieve drum maybe in the form of a cone or any other suitable body of revolution.

According to the inventiomeach scraper may have a porous or finely perforated part, behind the cutting edge, said part extending overthe entire'height of thescraper, and a pressure mediumcan besupplied to said part via the hollow scraper, said medium emerging uniformly over the entire surface of said part. In a construction of this kind, therefore, the friction along the'scraper is considerably reduced because the medium under pressure forms a thin film between the cake and the scraper. The speed and the pressure of the medium may be low so that standard blowers'and pumps can be used to deliver the medium. A'centrifuge of this type is very suitablefor the treatment of sensitive material because there-is. no direct contact betweenthe. scraper and the material during transport, so that there is little chance of damage to the material.

According to the invention, the scraper part situated behind the cutting edge may bemadecfrorn such a material over the entire heightof the scraper and may have such a smooth finish that the coefficient of friction between the said part and the cake is less thanthe coeff cient of friction'betweenthe drum surface and the cake. Acentrifuge'of thistypeis simpler to construct because it is not; necessary to, use any medium under pressure although in that case there islthejdisadvantag ethat the material of the cake which is to be transported has to move along the scraper surface, thus increasing the chance ,of mechanical damage.

According to the invention, the, scraper part situated behind the cutting edge may form an angle, over the entire height of the scraper in a plane-through the axis of rotation, with said axis. such angle being-larger than the angleformed by the'wall of the drum with the said axis, and the tangent of this angle between the scraper surface and the axis is, greater than the coefficient of friction-between thecake and the scraper surface. With a centrifuge of this type,:friction remains constant Finally, the scrap'ers constructed in the form of a hollow body may have a' channel which extends over the entire height, and the bottom of which forms the scraper part situated behind the cutting edge to allow the cake to slide off. With a construction of this type, there is good guidance of the material transported along the scraper, without excessive friction in the channel. The presence of the film of gas or liquid greatly reduces this friction. In the case of the film formed by the medium, this friction may be disregarded.

The invention will be explained in detail in the following description of one exemplified embodiment with reference to the drawing wherein:

FIG. 1 is a vertical section of a centrifuge according to the invention;

FIG. 2 is a-section on the line ll-II in FIG. 1;

FIG. 3 is a detail of FIG. 2 to an enlarged scale;

FIG. 4 is a diagram showing the equilibrium of forces on the cake in the drum;

FIG. 5 is a diagramshowing the equilibrium of forces on the cake on the scraper;

FIG. 6 is a diagram showing the equilibrium of forces on the cake on the scraper in a slightly modified embodiment.

A hub 2 is fastened on a shaft I and bears a centrifuge drum 4 by means of spokes 3. The inner wall of the drum 4 is formed by a sieve wall 5 constructed from rings or made from a perforate plate.

The liquid separated in the centrifuge is passed through small openings left between the rings, or through the perforations in the sieve wall' 5. The latter is supported by rings 6 which are provided onthe inside of the drum and which have passages. The drum extends conically so that the liquid passing through the ,sieve wall is conveyed downwards along the drum under the influence of centrifugal force. The liquid flows out via openings7 and along the edge 8 to a channel 9 on the inside of the casing 10. The channel 9 is provided with an outlet spigot 11 to discharge the liquid.

The shaft 1 is surrounded by a hollow shaft 12 which is guided on the shaft 1 by means of bearings 13 and 14. The axial forces are talten by an axial bearing 15. A drum 16 is connected to theshaft l2 by a baseplate 17. The drum 16 has a volute portion and a cylindrical portion. The volute portion 18 of the drum 16 has two chambers 19 at two diametrically opposite openings, the said chambers 19 extending over the entire height of the sieve wall 5 and having an outlet slot 20 near the sieve wall. By means of a wall 21 also secured to the drum 16, a chamber'22 is formed adjacent each chamber 19.

At the end adjacent the sieve wall 5, the chambers 22 are providedwith a scraper 23 which also extends over the entire height of the sieve wall. The scrapers 23 have a channelshaped cavity over the entire height, with a base 24 extending parallel to the sieve wall 5. Beneath the base 24, which consists of a porous material, there is a hollow space 25 which communicates via ducts 26 with the interior of the chamber 22. The latter communicates via apertures 27 with a chamber 28 situated around the shaft 1. A hose 30 is connected to the shaft 1 via a gland 29. Agas, for example air, at slight excess pressure, can be fedviathis hose to a bore 31 in the shaft 1, from which the gas is fedto chamber 28 via transverse bores 32.

Near the bottom end of the scraper 23, the baseplate 17 has openings through'which-the product removed by the scraper can drop down into the funnel 34 connected to the casing 10.

A number of sprays 35 are also secured to the drum 16 and co-rotate therewith and with the hollow shaft 12. The sprays 35 distribute a washing liquid over the centrifuged product and can be adjusted .so that the washing time can be varied with respect to the centrifuging time according to the required washing effect. The washing liquid can be fed to the sprays 35 in a manner not shown, via a gland 36.

A channel 38 leads into the cylinder part 37 of the drum 16 and the material to be centrifuged can be fed to the volute casing 18 via said channel.

The shaft 1, which can be driven by a motor, is suspended in a bearing block 39 supported in a rubber cushion 40. A gearbox 41 is secured to the bearing block 39 and can follow oscillations of the shafts 1 and 12. Shaft l bears a gearwheel 42 coupled via two intermediate gearwheels 43 and 44 to gearwhcel 45 secured .on the hollow shaft 12. The transmission ratio between the gcarwheels 42 and 45 is such that the speed difference between the shafts 1 and 12 is very small. This speed difference determines the centrifuging time. The latter can be adjusted as required by varying the transmission ratio of these gearwheels.

The centrifuge operates as follows: the mixture to be contrifuged is fed to the cylindrical chamber 37 via the channel 38 and then flows to the volute chamber 18 and is then deposited on the sieve wall 5 via the supply chambers 19 and the outlet apertures 20. The thickness of the layer deposited on the sieve wall 5 is determined by the width of the slots 20. Since the chambers 19 move slowly with respect to the sieve wall, material is continually deposited on the sieve wall 5 and is lifted off said wall after some time by the scraper 23 situated diametrically opposite.

As already described, the liquid emerges from the centrifuged mixture through the sieve wall 5 and passes to the discharge channel 9, from which the liquid is discharged via the spigot 11.

FIG. 4 illustrates the forces acting on a small portion 46 of the cake deposited on the sieve wall. in a plane through the axis ofthe drum, the drum wall forms an angle a with said axis. The centrifugal force C acts on part 46 in the outward direction and this force can be divided up into a component C,. directed at right angles to the sieve wall and a component C,, directed along the wall. The particle 46 is retained by the frictional force W acting along the wall. The frictional force W is equal to the force N extending at right angles to the wall, multiplicd by the coefficient of friction fhctwcen the wall and the particle 46. The force N is equal to C... The angle a is so selected that the coefficient of frictionfis equal to or slightly greater than tan a. This means that the particle 46 just continues to remain on the sieve wall 5 during the centrifuging process.

However, as soon as. particle 46 is lifted from the wall by the scraper 23 and reaches the part 24, the situation illustrated in FIG. 5 applies. The angle between the plane 24 and the axis of rotation is the same and thus the force C exerted by the centrifugal force is also the same. Since the gaseous medium emerges through the porous wall 24, the friction between the surface 24 and the particle 46 is substantially canceled out. The friction W is thus much less than the centrifugal force component C,, directed along the surface 24. This means that the particle 46 is no longer in equilibrium and slides down along the surface 24. This particle passes via opening 33 to funnel 34 from which it is then discharged.

FIG. 6 shows another embodiment. The surface 24 of the scraper in this case forms a larger angle with the axis of rotation than the sieve wall 5. This means that for a constant coefficient of friction the component C,, of the centrifugal force becomes larger than the opposing frictional force W. The particle 46 is again no longer in equilibrium and will slide down along the surface 24. The constructions shown in FIGS. and 6 can be combined. in which case the driving force C increases and the counteracting frictional force W decreases.

lclaim:

1. A centrifuge assembly for continuous recovery of solids from a liquid-solids mixture, comprising in combination:

a porous drum;

means for rotating said drum about its longitudinal axis;

' means for feeding a liquid-solids mixture to the inner surface of said drum whereby a cake of solids builds up upon said inner surface of said rotating drum;

scraper means within said drum for removing said cake and discharging the removed solids of the cake toward one end ofthe drum; and

means for rotating said scraper means at a speed slightly different from the speed-of said drum, said scraper means including a surface facing inwardly with respect to the axis of rotation and against which said removed solids of the cake are engaged by centrifugal force, said surface being inclined along its length outwardly with respect to its axis of rotation toward said one end of the drum by an amount sufficient to produce, due to its rotation, a component of centrifugal force acting on the removed solids of the cake parallel to said surface toward said one end of the drum which exceeds the frictional resistance to movement of said removed solids ulong suid surface due to the own ponent of centrifugal force acting normal to such surface whereby to feed the removed solids of the cake continuously therealong to a region outside said drum.

2. The centrifuge as defined in claim 1 wherein said inwardly facing surface of the scraper means is porous, and means for bleeding fluid under pressure through said surface to reduce the coefficient of friction of such surface.

3. A centrifuge for continuous recovery of solids from a liquid-solids mixture, comprising in combination:

a drum of porous material having a generally conical inner surface;

means for introducing a liquid-solids mixture into said drum for impingement against the inner surface thereof;

means for rotating said drum about its longitudinal axis to cause the solids to build up as a cake upon the inner surface of said drum;

scraper means within said drum for removing said cake and discharging the removed solids of the cake toward one end ofthe drum; and

means for rotating saidscraper means at a speed slightly different from the speed of said drum, said scraper means including a surface facing inwardly with respect to the axis of rotation and receiving the removed solids, such surfucc being inclined ulong its length outwurdly with respect to the axis of said drum towurd said one end ofthe drum sufficiently to cause migration of the removed solids therealong due to that component of the centrifugal force which is parallel to such surface.

4. The centrifuge as defined in claim 3 wherein the inclination of said surface of the scraper means is greater than the inclination of said inner surface of the drum with respect to its axis.

5. The centrifuge as defined in claim 3 wherein said surface of the scraper means is porous, and means for bleeding fluid under pressure through said surface to reduce the coefficient of friction between said solids and such surface.

6. A centrifuge for continuous recovery of solids from a liquid-solids mixture, comprising in combination:

a drum of porous material having a frusto-conical inner surface; means for feeding the liquid-solids mixture against the inner surface of said drum;

means forrotating said drum at a speed sufficient to hold the solids against theinner surface of said drum by a component of the centrifugal force acting normal thereto, while also producing a component of centrifugal force acting upon said solids which is parallel to said inner sur face of the drum. the coefficient of friction between said solids and the inner surface of said drum being sufficient to prevent axial migration of said solids relative to said drum whereby said solids tend to build up as a cake upon the inner surface of said drum; scraper means within said drum; and means for rotating said scraper means at a speed slightly different from the speed of rotation of said drum so that said scraper means moves slowly with respect to said drum continuously to remove said cake, and said scraper means including a surface portion extending axially within said drum and facing inwardly with respect'to the axis of rotation for receiving the cake as it is removed and said surface portion being inclined along its length in the same direction as the inner surface of said drum by an amount sufficient to produce a component of centrifugal force acting on the removed solids parallel to said surface which exceeds the frictional resistance to movement of said removed solids along said surface, to feed the removed solids outwardly of the drum under the influence of the centrifugal force acting thereon.

7. The centrifuge according to claim 6 wherein said coefficient of friction between said inner surface of the drum and said solids is just sufficient to prevent axial migration of said solids.

8. The centrifuge according to claim 7 wherein the coefficient of friction between said surface portion of the scraper means and said solids is less than the coefficient of friction between said solids and the inner surface of said drum.

9. The centrifuge according to claim 8 wherein said surface portion of the scraper means is inclined with respect to the axis of said drum at substantially the same angle as is the inner surface of said drum. 5

10. The centrifuge according to 'claim 8 wherein said surface portion of the scraper means is inclined with respect to the axis of said drum at a greater angle than is the inner surface of said drum.

11. The centrifuge according to claim 8 wherein said surface portion of the scraper means is porous, and means for bleeding air under pressure through said surface portion to reduce the coefficient of friction between said surface portion and said solids.

en-h 

